The present invention relates to a high-yield semiconductor laser device of real refractive index-guided structure operating at low current value, which is highly suitable for a light source for use in optical information processing and the like, and a method of the production thereof.
Below, conventional semiconductor laser devices will now be described.
As a light source for use in information processing of optical communication, optical disk, and the like, a single-mode light source is required, and hence semiconductor lasers with index-guided structure are employed. Especially, in recent years, a semiconductor laser device formed by means of metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE) excellent in film thickness uniformity as a crystal growth technique has been mainstream.
Below, a description will now be given to conventional semiconductor laser devices with index-guided structure which are implementable by a vapor phase growth technique. FIGS. 9 to 13 are sectional schematic diagrams of typical conventional semiconductor laser devices with index-guided structure. In the drawings, electrode layers formed on the top and bottom faces of each semiconductor substrate are omitted.
FIG. 9 shows a semiconductor laser device used in applications to optical disks Including a CD (See, JJAP, vol. 24, p.L89 (1985)). Referring to FIG. 9, on an n-type semiconductor substrate 11 made of gallium arsenide (GaAs), is formed an n-type cladding layer 12 made of gallium aluminum arsenide (GaAlAs). On the cladding layer 12, is formed an active layer 13 made of GaAlAs. On the active layer 13, is formed a p-type first cladding layer 14 made of GaAlAs. At the region other than a stripe region 15a serving as a current channel on the first cladding layer 14, is formed an n-type current blocking layer 15 made of GaAs for current confinement. On the first cladding layer 14 and current blocking layer 15, is formed a p-type second cladding layer 16A made of GaAlAs by an epitaxial growth technique. On the second cladding layer 16A, is formed a p-type contact layer 17 made of GaAs.
In a semiconductor device of this structure, the electric current injected from the contact layer 17 is effectively confined within the stripe region 15a due to the presence of the current blocking layer 15. This causes laser oscillation in the active layer 13 underneath the stripe region 15a. In this structure, the energy band gap of the current blocking layer 15 is smaller than the energy of wavelength of the laser light. Therefore, the laser light outside the stripe region 15a is absorbed in the current blocking layer 15. Consequently, the laser light is effectively confined within the stripe region 15a, resulting in a single-mode laser oscillation.
In the semiconductor laser device shown in FIG. 10, on an n-type semiconductor substrate 11, is formed an n-type cladding layer made of GaAlAs. On the cladding layer 12, is formed an active layer 13 made of GaAlAs. On the active layer 13, is formed a striped p-type cladding layer 16B. In the region other than the stripe region on the cladding layer 16B, a n-type current blocking layer 15 is formed, while a p-type cap layer 18 made of GaAs is formed on the stripe region on the cladding layer 16B. On the current blocking layer 15 and cap layer 18, is formed a p-type contact layer 17 made of GaAs.
The basic principle of operation of the semiconductor laser device shown in FIG. 10 is the same as that of the semiconductor laser device shown in FIG. 9. The presence of the current blocking layer 15 causes electric current and laser light to be confined within the stripe region, resulting in a single-mode laser oscillation (See, JJAP, vol.25, p.L498 (1986)).
The semiconductor laser device shown in FIG. 11 has a construction obtained by simplifying the semiconductor laser device shown in FIG. 10, and it has a ridge waveguide structure in which the current blocking layer 15 is not formed (See, SPIE, vol. 1043, p.61 (1989)). In this device, the reference numeral 19 in FIG. 11 denotes a dielectric film. The semiconductor laser device shown in FIG. 11 has problems of the occurrence of crack on cleavage, an increase in thermal resistance, and the like, due to the unevenness of its surface, and hence has not gone into mass production, whereas the device so constructed that its surface is flattened shown in FIG. 10 has found widespread application. In other words, the current blocking layer 15 also has an effect of flattening the surface of the semiconductor laser device to enhance the productivity in large quantity.
In recent years, there has also been developed a semiconductor laser of real refractive index-guided structure employing a GaAlAs current blocking layer (Japanese Laid-Open Patent Publication No. 62-73687). With this structure, a current blocking layer is formed so as to have a lower refractive index than that of a cladding layer, resulting in the confinement of laser light within a stripe region. Accordingly, unlike the devices of the constructions shown in FIGS. 9 and 10 in which laser light is confined by light absorption of the current blocking layer 15, a single-mode semiconductor laser device operating at low current value can be obtained due to low internal loss.
FIG. 12 illustrates a semiconductor laser device of a buried hereto (BH) structure applied to the field of optical communications and the like (See, IEEE. J. Quantum Electron., QE-16, p205 (1980)). Referring to FIG. 12, the reference numerals 20 and 21 indicate a high-resistivity layer and a zinc diffusion region, respectively. In the semiconductor laser device shown in FIG. 12, the high-resistivity layer 20 on both sides of the active layer 13 serves as a current blocking layer, which causes abrupt confinement of laser light within the stripe region due to the difference in refractive index between the active layer 13 and high-resistivity layer 20. Abrupt optical confinement increases the optical density in the active layer 13. Therefore, this structure is inadequate to obtain high output power, and hence commonly applied to low output power semiconductor laser devices in practice use.
FIG. 13 illustrates an example of conventional devices wherein on a cladding layer 12 is formed an optical guiding layer 22, and on the optical guiding layer 22 is formed a striped active layer 13 (See, IEEE, J. Quantum Electron., QE-15, p451 (1979)). The semiconductor laser device of this structure has a disadvantage in that when the active layer 13 is etched in stripe, the active layer 13 is exposed in air to cause reliability degradation. Also, a current blocking layer 28 made of GaAlAs is formed on the uppermost surface of grown layers, resulting in broad spread of current. This entails a problem of raising the threshold value, not yet leading to the practical application thereof.
On the other hand, there has been a demand for a semiconductor laser device which causes blue laser light to oscillate as a light source for high density information recording adaptable to a multimedia. However, a practical semiconductor laser device which causes blue laser light to oscillate has not yet been implemented.
As one of the materials for implementing this semiconductor laser device, a semiconductor material of the GaN system whereby, for example, an active layer is made of InGaN has attracted attention. However, the semiconductor material of the GaN system has high atomic bond strength as compared with GaAs or the like, and hence it is difficult to be etched.
As described above, semiconductor laser devices of conventional structures are so constructed that current and laser light are confined into a stripe by the use of a current blocking layer. Therefore, in producing the semiconductor laser devices, a step for etching a current blocking layer in stripe, or a step for selectively forming a current blocking layer outside the stripe region is indispensable.
As the thickness of a current blocking layer required for blocking injection current, approximately the diffusion length for electrons or holes is necessary. Therefore, an n-type current blocking layer made of GaAs is required to have a thickness of about at least 0.5 to 1 .mu.m, while a p-type current blocking layer made of GaAs is required to have a thickness of at least 2 to 3 .mu.m. In producing a semiconductor laser device, the thinner a current blocking layer is, the easier the production thereof becomes. Accordingly, an n-type current blocking layer is employed in practical use. The formation of the n-type current blocking layer requires the etching of a stripe region to a depth of about 1 .mu.m. However, deep etching causes a variation in stripe width after etching, resulting in a lowered yield. The stripe width after etching is required to be controlled to, for example, about 2.+-.0.2 .mu.m in the case of real refractive index-guided structure. In a semiconductor laser device in which light distribution is directly affected by the stripe width, the control of the stripe width is very important.
The problem of controlling a stripe width arises especially when an etching stopping layer is provided in order to each the blocking layer selectively in a direction of depth with stability. This is attributable to, for example, the fact as follows: in FIG. 9, even if an etchant capable of selectively etching the current blocking layer 15 is used, when the thickness of the current blocking layer 15 is large, the film thickness thereof also exhibits a wide range of variation. Therefore, side etching during the time required for removing all the regions to be removed in the current blocking layer 15 causes a wide range of variation in stripe width.
Specifically, when the variation of the thickness of the current blocking layer 15 in a wafer is .+-.10%, the thickness of the current blocking layer 15 is 1.+-.0.1 .mu.m in the wafer. This results in a variation of .+-.0.2 .mu.m in stripe width due to side etching even if the etch rates of side etching and of etching in a direction of depth are the same with each other. Actually, the variation in mask width caused by a photolithography process is further added to the above-described variation. This entails a problem that the value of 2.+-.0.2 .mu.m in the above-described example of design cannot be satisfied, resulting in a lowered yield.
As in the construction shown in FIG. 10, even when the current blocking layer 15 is selectively formed outside the stripe region, a process for etching a p-type cladding layer 16B to a thickness of 1 .mu.m is required, resulting in the same difficulty in controlling of stripe width as that described above. In other words, as long as the current blocking layer 15 is necessary, the problem of controlling stripe width by deep etching cannot be avoided. However, in the construction shown in FIG. 9, a process of selectively growing the current blocking layer 15 on the first cladding layer 14 can be considered as the production process. This requires the following steps. That is, prior to selective growth, a dielectric film such as nitride film serving as a mask for selective growth is formed on the first cladding layer 14 within the stripe region 15a by means of a technique such as plasma-activated chemical vapor deposition (PCVD). After selective growth, the above-described dielectric film is removed by means of a technique such as reactive ion etching. These steps entail problems of causing a large quantity of crystal defect to be introduced in the first cladding layer 14 in close to the light emitting region of the active layer 13, and complicating the production method, thereby not yet leading to the implementation.
On the other hand, as one of the materials for implementing a semiconductor laser device which emits blue laser light, semiconductor materials of the GaN system whereby, for example, an active layer is made of InGaN has attracted attention. However, the semiconductor material of the GaN system has high atomic bond strength as compared with GaAs or the like, and hence it is difficult to be etched. This is one of the causes for preventing the implementation of the semiconductor laser device. That is, the formation of waveguide in the inside of the semiconductor laser device requires etching by means of wet etching technique which will not cause damage to crystal. However, an etchant suitable for use in this step has not yet been found.
Even if there is a suitable etchant, there occurs a problem as follows: the GaN crystal epitaxial growth is generally of hexagonal system unlike crystal of the cubic system such as GaAs system. When subjected to deep etching required for the above-described conventional constructions, the etched surface becomes uneven. This leads to large loss in waveguide, resulting in no possibility of laser oscillation. Therefore, even if etching is performed, there is a limitation that only extremely shallow etching is allowed.